1. Field of the Invention
The present invention generally relates to electroacoustic transducers and more particularly to a bass-reflex type electroacoustic transducer having two ducts.
2. Description of the Prior Art
As a prior-art sound emanating device, a bass-reflex type cabinet is widely used, in which while an inner volume of a cabinet incorporating therein a speaker is kept constant, a low band of a reproduced sound is extended by decreasing a threshold frequency f.sub.L. FIG. 1 schematically illustrates an example of such bass-reflex type cabinet which is generally represented by reference numeral 10 therein.
As shown in FIG. 1, in the bass-reflex cabinet 10, a duct or port 11 is provided in a speaker mount surface 10f, whereby a phase of an acoustic wave emanated to the rear side of a diaphragm of a speaker unit 1 is inverted by equivalent mass of the port 11 and stiffness presented by air within the cabinet 10 so as to be emanated with an acoustic wave emanated from the front of the speaker 1 in the same phase as each other.
L (cm) assumes the length of the port 11, S (cm.sup.2) assumes the cross sectional area of the port 11 and a (cm) assumes the effective radius of the speaker unit 1. Then, the equivalent mass m (gram) of the port 11 is expressed by the following equation (1): ##EQU1## where L.sub..theta. =L+0.96.sqroot.S is established.
For example, for the speaker unit 1 having an aperture with the diameter of 6.5 cm and an effective radius a =2.5 cm, the length L.sub.1 1 and the cross sectional area S.sub.1 1 of the port 11 are determined as follows:
L.sub.1 1 =5.5 cm and S.sub.1 1 =3.23 cm.sup.2 PA1 L.sub..theta.1 1 =7.2 cm and m.sub.1 1 =1.03 g
In that case, the effective length L.sub..theta.1 1 and equivalent mass m.sub.1 1 of the port 11 take the following values:
A sound pressure versus frequency characteristic thereof is represented in FIG. 2.
Further, the bass-reflex type electroacoustic transducer employs the single port as shown in FIG. 1 and it is frequently observed that two ports whose resonance frequencies are set to low and middle sound bands as described in Japanese Laid-Open Utility Model Gazette No. 53-4929.
While the bass-reflex cabinet incorporating therein the speaker is presented as described above, a bass-reflex type headphone or earphone and the like are known, in which the port 11 is provided at the rear portion of a housing 8 of a headphone unit 9 having an ear pad load 7 provided in front of a diaphragm 4 as shown in FIG. 3. Also in such headphone, equivalent mass m (gram) thereof is designed so as to satisfy the equation (1) when the effective radius of the diaphragm 4 of the headphone unit 9 is taken as a (cm), the length of the port 11 is taken as L (cm) and the cross sectional area of the port 11 is taken as S (cm.sup.2). FIG. 4 shows a sound pressure versus frequency characteristic where the length and cross section area of the port 11 are L.sub.1 1 =12 cm and S.sub.1 1 '=7 mm.sup.2 for the diaphragm whose aperture is 3 cm in diameter and of which the effective radius a is 15 mm.
As is clear from the aforenoted equation (1), if the cross section area S of the port 11 is determined small, then a predetermined equivalent mass m can be obtained regardless of the short length L. In that case, however, air resistance of the port 11 is increased and air flow velocity is increased, thus resulting in a so-called wind noise being increased. To avoid this disadvantage, the port 11 having the sufficient cross section area and length is employed as represented in the example of the aforenoted numerical values.
Nevertheless, if the length of the port 11 is increased, then resonance and antiresonance of air within the port 11 occur in the middle and high sound bands. There is then the substantial disadvantage that tone quality of a reproduced sound will be deteriorated.
If the following equation (2) is established as EQU L.sub..theta. =2 n.multidot..lambda./4 (2)
where .lambda. (cm) represents the wavelength of the reproduced sound and n represents the natural number, resonance occurs so that acoustic impedance of the port 11 seen from the inside of the cabinet 10 is minimized. Consequently, as shown in FIGS. 2 and 4, a peak 6 appears in the middle and high bands of the sound pressure versus frequency characteristic.
If the following equation (3) is established as EQU L.sub..theta. =(2n-1).lambda./4 (3)
antiresonance occurs so that acoustic impedance of the port 11 seen from the inside of the cabinet 10 and the housing 8 is maximized. Thus, as shown in FIGS. 2 and 4, a dip 5 appears in the middle and high bands of the sound pressure versus frequency characteristic.
FIGS. 2 and 4 illustrate the sound pressures measured just in front of the outlet of the port 11.
A resonance frequency f.sub.R and an antiresonance frequency f.sub.A of the port 11 are expressed by the following equations (4) and (5) EQU f.sub.R =2n c/4 L.sub..theta. ( 4) EQU f.sub.A =(2 n-1) c/4 L.sub..theta. ( 5)
where c represents the sound velocity.
In the aforenoted example of numerical values, the resonance frequencies f.sub.R of 2.lambda./4, 4.lambda./4 . . . modes and the antiresonance frequencies f.sub.A of 1.lambda./4, 3.lambda./4 . . . modes are provided as represented on table 1 with respect to the speaker system and are provided as represented on table 2 with respect to the headphone system. It will be seen in tables 1 and 2 that those resonance frequencies f.sub.R and antiresonance frequencies f.sub.A correspond with frequencies at the peaks 6 and the dips 5 shown in FIGS. 2 and 4.
TABLE 1 ______________________________________ n f.sub.A (kHz) f.sub.R (kHz) ______________________________________ 1 1.18 2.36 2 3.54 4.72 3 5.90 7.08 4 8.26 9.54 ______________________________________
TABLE 2 ______________________________________ n f.sub.A (kHz) f.sub.R (kHz) ______________________________________ 1 -- 1.38 2 2.07 2.76 3 3.45 4.15 ______________________________________
Accordingly, it is a general object of the present invention to provide an improved electroacoustic transducer having two ducts which can eliminate the aforenoted short comings and disadvantages of the prior art.
More specifically, it is an object of the present invention to provide an electroacoustic transducer having two ducts which can prevent tone quality of a reproduced sound from being deteriorated due to resonance and antiresonance of ports.
It is another object of the present invention to provide an electroacoustic transducer having two ducts which is suitably applied to a bass-reflex speaker system.
It is still another object of the present invention to provide an electroacoustic transducer having two ducts which is suitably applied to a bass-reflex headphone.
It is a further object of the present invention to provide an electroacoustic transducer having two ducts which is suitably applied to a bass-reflex earphone.
According to an aspect of the present invention, an electroacoustic transducer apparatus is comprised of an electroacoustic transducing element supplied with an input signal, a housing in which the electroacoustic transducing element is attached, and a plurality of sound ducts communicating the inside and outside of the housing, wherein the sound ducts are different in length and are equal in equivalent mass.
The above, and other objects, features and advantages of the present invention will be apparent in the following detailed description of the preferred embodiments when read in conjunction with the accompanying drawings, in which like reference numerals are used to identify the same or similar parts in the several views.